1. Field of the Invention
This invention relates to determining the relative positions of two or more objects; and, more particularly, to radiating field from each object, detecting the field at a second object and analyzing the field to determine the position of the radiating object, and, if desired, the position of a third object.
2. Description of the Prior Art
The use of orthogonal coils for generating and sensing magnetic fields is known. Such apparatus, for example, has received wide attention in the area of mapping magnetic fields to provide a better understanding of their characteristics. If a magnetic field around generating coils can be very accurately mapped through use of sensing coils, it has also been perceived that it might be possible to determine the location of the sensing coils relative to the generating coils based on what is sensed. However, a problem associated with doing this is that there is more than one location and/or orientation within a usual magnetic dipole field that will provide the same characteristic sensing signals in a sensing coil. In order to use a magnetic field for this purpose, additional information must therefore be provided.
One approach to provide the additional information required for this purpose is to have the generating and sensing coils move with respect to each other, such as is taught in U.S. Pat. No. 3,644,825, issued Feb. 22, 1972. The motion of the coils generates changes in the magnetic field, and the resulting signals then may be used to determine direction of the movement or the relative position of the generating and sensing coils. While such an approach removes some ambiguity about the position on the basis of the field sensed, its accuracy is dependent on the relative motion, and it cannot be used at all without the relative motion.
The Kalmus approach is somewhat different in his U.S. Pat. No. 3,121,228, entitled DIRECTION INDICATOR, issued Feb. 11, 1964. In this patent, one object is equipped with perpendicular transmitter coils excited by phase-quadratured alternating current. The other object is equipped with similarly positioned receiver coils, the phase difference between the voltages induced in the receiver coils varying with the angle between the axes of the transmitter and receiver coils and the sum of the voltages induced in the receiver coils indicating the distance between the objects. The apparatus disclosed in this patent, however, is capable of providing two-dimensional distance and direction only, is limited to near-field ranges and provides non-ambiguous information only so long as the relative physical rotation between the objects does not exceed 90 degrees. See, also, Kalmus, A New Guiding and Tracking System, IRE Transactions on Aerospace and Navigational Electronics, March 1962, pages 7-10.
U.S. Pat. No. 3,868,565, issued Feb. 25, 1975, to the instant inventor, teaches a tracking system for continuously determining at the origin of a reference coordinate system the relative translation and orientation of a remote object. The tracking system includes radiating and sensing antenna arrays each having three orthogonally positioned loops. Properly controlled excitation of the radiating antenna array allows the instantaneous composite radiated electromagnetic field to be equivalent to that of a single loop or equivalent stub antenna oriented in any desired direction. Further, control of the excitation causes the radiated field to nutate about an axis denoted a pointing vector.
The tracking system is operated as a closed loop system with a computer controlling the radiated field orientation and interpreting the measurements made at the sensing antenna array. That is, an information feedback loop from the sensing antenna array to the radiating antenna array provides information for pointing the axis of the nutating field toward the sensing antenna array. Accordingly, the pointing vector gives the direction to the sensing antenna array from the radiating antenna array. The proper orientation of the pointing vector is necessary for computation of the orientation of the remote object. The signals detected at the sensing antenna include a nutation component. The nutating field produces a different nutation component in the signals detected in each of the three orthogonal loops of the sensing antenna array. The orientation of the sensing antenna array relative to the radiated signal is determined from the relative magnitudes and phase of these modulation components.